Unless you've spent a lot of time grinding, the Seacomers is a purely defensive level. The enemy units are vast in number and generally of an advanced class and decent level. Taking the fight to them isn’t much of an option. Your army will spawn in the southern part of the map. You will already be surrounded with a wave of reinforcements moving in from the enemy ship in the north. The key to winning this level is to take advantage of the environment. The map is loaded with natural choke points that, used properly, can pretty well negate the enemy army’s superior numbers.

Place your strongest units at the opening just above where your party spawned at. Make sure they are equiped with weapons to take out armored units. Magic users, Chrom with Rapier, Vaike with Hammer. Units like these with a high defense are most effective here. Cherche can handle the two knights on the right so send her out there and make sure she is equipped with a hammer. Just make sure only the two knights on the right can attack her. On the left don't advance just have a couple of strong units good against units on horseback just within the range of the enemy to lure them out and then when they attack finish them all off with the majority of your party next turn. After the enemy attacks a large portion of the enemy units should be gone so you can advance slowly with your armor slaying units at the front. Best to split your party into two groups at this point. Both groups need units good with units on horseback so Chrom is effective here. Most of the units have swords and lances and are farily slow so your quicker units have the advantage. Once you lure out the cavaliers and paladins on the boat and defeat them lure out the boss and gang up on him with your party and he'll be an easy victory.